1. Field of the Invention
The present invention relates to a focus detecting apparatus used for optical equipment such as a single lens reflex camera.
2. Related Background Art
The phase difference detection method is traditionally known as a focus detecting method in a single lens reflex camera. The method is explained with reference to FIG. 9.
The luminous flux, which enters through a lower domain 21a of an objective lens 21, passes through a visual field mask 31, a field lens 32, an aperture 33a and a re-imaging lens 34 in the order in which they are listed before it forms an image on an upper image sensor array 41a. Similarly, the luminous flux, which enters through an upper domain 21b of the objective lens 21, passes through the visual field mask 31, the field lens 32, an aperture 33b and a re-imaging lens 35 in the order in which they are listed before it forms an image on a lower image sensor array 41b.
In the so-called "before-focus" state where the objective lens 21 forms the sharp image of a subject before an expected focusing surface, the pair of subject images formed on the image sensor arrays 41a and 41b move away from each other. In contrast with this, in the so-called "behind-focus" state wherein the sharp image of a subject is formed behind an expected focus surface, the pair of subject images move closer toward each other. At the time of focusing when the objective lens 21 forms the sharp image of a subject on the expected focus surface, the subject images on the image sensor arrays 41a and 41b show relative coincidence. Therefore, the focus adjusting condition of the objective lens 21, that is, the deviating amount and the deviating direction (hereinafter referred to as "defocus amount DF") in this case can be known by determining the relative positions of the pair of the subject images by converting the pair of the subject images into electrical signals using the image sensor arrays 41a and 41b which perform photoelectric conversion, then subjecting those signals to arithmetic operation executed by a microcomputer not shown in the accompanying drawings.
Next, the procedure of the arithmetic processing by a microcomputer for determining the de-focus amount is discussed.
Each of the pair of the image sensor arrays 41a and 41b consists of a plurality of photoelectric converting devices which provide a plurality of photoelectric conversion output a1 . . . an, and b1 . . . bn as shown in FIGS. 10A and 10B. Then, each string of data is relatively shifted by a prescribed quantity of data (hereinafter referred to as "shift amount" and shown by a symbol "L") to perform relative arithmetic operation. In this case, relative amount C (L) is computed from the equation below: ##EQU1##
A shift amount L is an integral value, and the first term k and last term r may increase or decrease in accordance with the shift amount L. The shift amount L, which produces a relative amount C(L) with a minimal value among the relative amounts C(L) thus obtained, is multiplied by a constant Kf determined by the pitch width of the photoelectric converting devices of the image sensor array 41 and focus detecting optical systems, 31, 32, 33, 34 and 35 shown in FIG. 9. The result is a defocus amount DF.
Relative amounts C(L), however, take discrete values as shown in FIG. 10C. The minimum unit of a detectable defocus amount DF is restricted by the pitch width of the photoelectric converting devices of the image sensor arrays 41a and 41b. Therefore, a method for carrying out close focus detection by performing interpolatory arithmetic operation based on discrete relative amounts C (L) and then calculating a minimal value Cex was disclosed by the present applicant under Japanese patent laid-open application No. 60-37513. This method is used to calculate minimal value Cex according to relative amount C(0) which is a minimal value and relative amounts C(1) and C(-1) on the shift amount L on both sides as shown in FIG. 10D. A shift amount Fm which gives the minimal value Cex and defocus amount DF are determined by EQU DF=Kf.times.Fm EQU Fm=L+DL/E
where EQU DL={C(-1)-C(1)}/2 EQU Cex=C(0)-DL EQU E=MAX {C(1)-C(0), C(-1)-C(0)} (2)
In the equation above, MAX {A, B}shows that A or B, whichever is greater, is selected.
It is necessary to determine whether the defocus amount DF is a reliable value or a value affected by the fluctuation in the relative amount due to noises or other cause. The defocus amount DF is judged to be reliable if the following condition is satisfied: EQU E&gt;E1 and Cex/E&lt;G1 EQU where E1 and G1 are certain prescribed value. Condition (1)
E is a value that depends on the contrast of a subject, and the greater the value, the higher the contrast becomes, resulting in higher reliability. Cex/E primarily depends on noise components, and as it becomes closer to 0, the reliability improves. If the defocus amount DF has been judged to be reliable from the computation result of Condition (1), then the objective lens 21 is driven to move to the focusing position in accordance with the defocus amount DF.
In the focus detection method described above, reliable focus detection cannot be achieved unless the subject image formed on the image sensor array 41 has a contrast of a certain level or higher. Generally, in a photographing subject, the horizontal contrast is frequently stronger than vertical contrast. For this reason, a pair of the image sensor arrays 41a and 41b is positioned horizontally against a photographing screen to perform focus detection by the horizontal contrast.
Another method is also known, which permits accurate focus detection for both horizontal and vertical contrasts. In this method, a pair of image sensor arrays 43a and 43b is oriented horizontally, and another pair of image sensor arrays 43c and 43d are oriented vertically as shown in FIG. 11, considering a case where the horizontal contrast is low while the vertical contrast is high, or a case where a camera is positioned vertically.
The optical system in which a pair of image sensor arrays is positioned horizontally and another pair is positioned vertically to perform focus detection as discussed above is shown in FIG. 12A.
A visual field mask 36, a field lens 37, an iris 38, a re-imaging lens 39 and an image sensor array 42 are positioned on the optical axis of an objective lens 22 as illustrated. The visual field mask 36 has a cross-shaped aperture and it is located in the vicinity of the expected focus surface of the objective lens 22 to control the aerial image of a subject formed by the objective lens 22. The iris 38 has four apertures, 38a, 38b, 38c and 38d.
The re-imaging lens 39 consists of four lenses, 39a, 39b, 39c and 39d as shown in FIG. 12B, and it serves to form the image restricted by the visual field mask 36 onto the image sensor array 42. Therefore, the luminous flux entering through a domain 22a of the objective lens 22 goes through the visual field mask 36, the field lens 37, the aperture 38a of the iris 38 and the lens 39a of the re-imaging lens 39 in the order in which they are listed before it forms the image on the image sensor array 42a. Similarly, the luminous fluxes entering through domains 22b, 22c and 22d of the objective lens 22 form images on image sensor arrays 42b, 42c and 42d. The subject images formed on the image sensor arrays 42a 4 and 42b move apart from each other if the objective lens 22 is in the before-focus condition, or move closer toward each other in the behind-focus condition. The images are arranged with a certain distance when focusing is achieved. Therefore, the focus adjusting condition of the objective lens 22 in the horizontal direction can be detected by subjecting the signals from the image sensor arrays 42a and 42b to the arithmetic operation executed by a microcomputer. Similarly, the individual subject images formed on image sensor arrays 42c and 42d move farther apart from each other when objective lens 22 is in the before-focus condition, while they move closer to each other in the behind-focus, and they are positioned with a certain distance between them when focusing is accomplished. Thus, the focusing adjusting condition of the objective lens 22 in the vertical direction can be detected by subjecting the signals from the image sensor arrays 42c and 42d to the arithmetic operation.
Some of the known methods for deciding whether the detection result from the horizontal focus adjusting condition or that from the vertical focus adjusting condition should be used to drive the lenses include:
(1) One in which the detection result with higher reliability is selected. (For instance, the result in which the value of E mentioned above is larger), PA1 (2) One in which the priority is given to one direction (vertical direction, for example), and if no reliable result is obtained or if no minimal value C(0) mentioned above exists, making arithmetic operation impossible, then focus detection in the other direction (vertical direction) is performed, and PA1 (3) One in which the mean value of the computing results from both directions is taken.
In such focus detecting apparatus, when the images of a plurality of subjects with different distances are formed on image sensor arrays, there are possibilities of making misjudgment on proper focusing as a result of adopting the average distance of those subjects or of focus detection being made impossible. To solve the problem, a camera has been proposed, which is designed to perform focus adjustment in a way that a subject image is subdivided by dividing a pair of image sensor arrays into a plurality of blocks, and the arithmetic operation for focus detection is executed on each of such blocks. From the arithmetic operation results obtained, a block which includes, for example, a subject existing at the closest point or a subject with the strongest contrast is found, and focus adjustment is performed in accordance with the computation result of the block. (Refer, for instance, to U.S. Pat. Nos. 4,687,917 and 4,851,657.)
In the present invention, to divide the image sensor arrays into a plurality of blocks, for instance, a fixed number of the first term k and last term r in the shift amount L=0 is equation (1) described above is provided. For example, as shown in FIG. 13A, to divide the image sensor array into 6 blocks, the relative amounts C(L) at block 1 are calculated using the equation (1) with k=1 and r=6 when the shift amount L=0, then the defocus amount DF is calculated using equation (2) according to the calculation results of equation (1). Similarly, at blocks 2, 3, 4, 5 and 6, setting is made to provide (k=7, r=12), (k=13, r=18), (k=19, r=24), (k=25, r=30), and (k=31, r=36) when the shift amount L=0, then the defocus amount DF is calculated using the equations (1) and (2). In FIG. 13B, there are 3 blocks, the width of each block being greater than that shown in FIG. 13A.